The present invention generally relates to image display units and methods of producing same. More specifically, the present invention relates to an image display unit including light emitting devices and methods of producing same.
Various lightweight and thin display units have been developed, including LED (Light Emitting Diode) displays, liquid crystal displays, and plasma displays. The application of these image display units has been extended along with progress of computer techniques. For example, display units having a diagonal size of about 30 to 150 cm have been used for television receivers, video reproducing devices, and output units for game devices, and further, display units having a diagonal size smaller than about 30 cm have been used for vehicle-mounted navigation systems, picture recording systems, and monitors.
Each of the above-described image display units, however, has drawbacks in terms of characteristics such as resolution, luminance, light output/power efficiency, and image quality, and further, in terms of screen size and production costs. For example, in display units of a type using light emitting diodes arrayed in a matrix, individual light emitting diodes are collectively mounted to form an array of the light emitting devices. However, since each light emitting diode is packaged, it has a size as large as several millimeters. In general, a set of light emitting diodes of differing colors (e.g., red, green and blue) make up a pixel, which is the basic unit of the composition of an image on an image display unit. Thus, the size of each pixel becomes large, thereby degrading the resolution (i.e., the smaller the pixel size, the higher and better the resolution). Additionally, since the cost per each pixel is raised, the production costs become high, in particular, for an image display unit having a large screen.
In liquid crystal display units, a substrate forming part of the display unit is put in a film formation apparatus, kept in vacuum, and devices such as transistors and wiring are formed using photolithography. In such display units, particularly, when increasing the resolution of the liquid crystal display, a process control must be performed on the order of xcexcm. Accordingly, to improve the production yield, the process control must be strictly performed, and therefore, the production costs are increased when producing a liquid crystal display unit having a large screen. Further, liquid crystal display units have a viewing angle dependence in which the contrast or tint varies depending on a viewing angle, and also experience a response speed delay when one color is changed to another color.
Plasma display units primarily function by making use of a mechanism that generates a discharge in a narrow space on the order of a pixel size and visual light is generated by exciting phosphors with the aid of ultraviolet rays derived from an ionized gas generated from the discharge. Accordingly, plasma display units have low luminous efficiency and large power consumption. Further, external light is reflected from phosphors, thereby degrading the contrast. Additionally, plasma display units have a narrow color reproduction range.
Accordingly, each of the above-described image display units make it difficult to form a large-sized screen, are high in production costs, and have problems in terms of resolution, process control, image quality, and luminous efficiency.
Production costs for image display units using LEDs can be reduced by producing a number of LEDs from one wafer. More specifically, for example, the production costs of an image display unit can be reduced by separating an LED chip having a larger area into LED chips each having a significantly smaller area and mounting the LED chips, thus separated, on a board.
In this regard, there are various techniques known in which devices formed at a high density are moved to a wide region while being spaced from each other by transfer or the like, to obtain a relatively large display unit such as an image display unit. For example, U.S. Pat. No. 5,438,241 discloses a thin film transfer method, and Japanese Patent Laid-open No. Hei 11-142878 discloses a method of forming a transistor array panel for a display unit.
In the transfer method disclosed in U.S. Pat. No. 5,438,241, devices densely formed on a substrate are coarsely re-arrayed on a specific display panel by transferring the devices densely formed on the substrate to a stretchable board provided with an adhesive layer, extending the stretchable board along a first axis and then along an orthogonal axis while monitoring the spacing between the devices along both axes, and transferring the devices on the extended stretchable board onto the display panel.
In the technique disclosed in Japanese Patent Laid-open No. Hei 11-142878, thin film transistors forming a liquid crystal display portion on a first substrate are all transferred on a second board, and the thin film transistors are selectively transferred from the second board to a third board with an array pitch corresponding to a pixel pitch (i.e., the distance from center to center of any two adjacent pixels).
The above-described techniques, however, encounter the following problems. The transfer method disclosed in U.S. Pat. No. 5,438,241, in which devices closely formed on a substrate are coarsely re-arrayed on a display panel requires that the device position is deviated by a chip size (e.g., about 20 xcexcm), at a minimum, depending on at which position of an adhesively bonding surface of the device chip, a fixed point (supporting point) at the time of extension of the stretchable board is located. As a result, this transfer method requires accurate positional control for each device chip. Accordingly, when forming a high definition TFT array panel requiring positional accuracy of at least about 1 xcexcm, it takes a lot of time to perform positioning of the TFT device chips including positional measurement and control for each TFT device chip. Another disadvantage of this transfer method is that when transferring TFT device chips on a resin film having a large thermal expansion coefficient, positional accuracy may be reduced depending on variations in temperature and stress, both before and after the positioning operation. Thus, from the viewpoint of mass-production, this transfer method has problems in terms of positional accuracy and time constraints.
The technique disclosed in Japanese Patent Laid-open No. Hei 11-142878 has the following problem. In this method, wiring electrodes and the like are formed after final transfer. However, since it has been required to reduce sizes of devices such as thin film transistors or light emitting devices for satisfying a requirement toward high integration of the devices so as to realize high-speed operation and reduction in costs, if a wiring layer and the like are formed after the devices are arrayed with an array pitch corresponding to a specific pixel pitch, then it is required to form wiring in a state that the micro-chips are already arrayed in a wider region. As a result, this method has a problem in terms of possible wiring failures due to problems with the positional accuracy of the devices.
There have been known some image display units in which light emitting devices such as light emitting diodes are mounted so as to be arrayed on a wiring board in a matrix. Japanese Patent No. 2895566 and Japanese Patent Laid-open No. Hei 9-293904 disclose light emitting diodes of a so-called flip-chip type. When producing an image display unit by arraying such light emitting diodes in a matrix, each light emitting diode must be contained in a package and an array of a number of these light emitting diodes must be mounted for assembly into a flat type image display unit or the like. Thus, since LEDs formed on a wafer are separated into individual chips and are each sealed in a package, each LED chip in a bare chip state has a size less than about 1 mm (e.g., each side of an approximately square-shaped chip is less than about 1 mm) and the package of the LED chip has a size on order of about several millimeters. As a result, the size of one pixel becomes large, thereby resulting in resolution degradation, and failing to produce a small-sized high definition image display unit. Further, for a light emitting diode made from a GaN based nitride semiconductor, since the light emitting diode is generally formed on a sapphire substrate, the package of each LED becomes thicker than the thickness of the sapphire substrate.
In view of the foregoing, a need exits to provide an image display unit capable of enhancing characteristics such as resolution, image quality, and luminous efficiency, facilitating formation of a large-sized screen, and reducing the production time and costs. An additional need exists to provide a method of arraying devices, which is capable of transferring micro-devices to a wider region without degrading positional accuracy after transfer and without the occurrence of a wiring failure.
The present invention provides an improved image display unit including light emitting devices and methods of producing same. In this regard, the present invention provides improved image display units having enhanced characteristics such as resolution, image quality, and luminous efficiency, while facilitating formation of a large-sized screen, and reducing the production time and costs. Additionally, the present invention provides a method of arraying devices, for example, to be used in an image display unit, which enables transferring micro-devices to a wider region without degrading positional accuracy after transfer and without the occurrence of a wiring failure.
To this end, in an embodiment of the present invention, a method of re-arraying a number of devices arrayed on a first substrate each having a first pitch onto a second substrate is provided. The method includes the steps of transferring the devices to a temporary holding member such that the devices are spaced apart, each having a second pitch, wherein the second pitch is larger than the first pitch, holding the devices on the temporary holding member, and transferring the devices to the second substrate such that the devices are spaced apart, each having a third pitch, wherein the third pitch is larger than the second pitch.
In an embodiment, the second pitch is about an integer multiple of the first pitch, and the third pitch is about an integer multiple of the second pitch.
In an embodiment, after the step of transferring the devices to the temporary holding member, the method further includes the steps of molding the devices with a resin, forming electrodes of the devices on the resin, and processing the resin to divide the resin into a number of sections.
In an embodiment, the step of transferring the devices to the temporary holding member includes selectively transferring from the first substrate the devices located at positions spaced from each other when the first substrate is opposed to the temporary holding member.
In an embodiment, the step of transferring the devices to the second substrate includes selectively transferring from the temporary holding member the devices located at positions spaced from each other when the temporary holding member is opposed to the second substrate.
In an embodiment, the devices are transferred from the temporary holding member to a position adjacent to each other on the second substrate that is different from a position of the devices on the temporary holding member.
In an embodiment, the step of transferring the devices to the temporary holding member and the step of transferring the devices held on the temporary holding member to the second substrate are performed employing at least one of a mechanical mechanism and an optical mechanism.
In an embodiment, the mechanical mechanism is capable of selectively transferring the devices while imparting a dynamic energy to each of the devices.
In an embodiment, the mechanical mechanism is capable of transferring the devices by selectively attracting the devices.
In an embodiment, the optical mechanism is capable of selectively transferring the devices while imparting a light energy to each of the devices by light irradiation.
In an embodiment, the first substrate is a translucent substrate.
In an embodiment, each of the devices is a semiconductor device including a nitride semiconductor and the light irradiation is performed using a laser beam.
In an embodiment, at least a portion of the devices are selected from the group consisting of a light emitting device, a liquid crystal device, a photoelectric transfer device, a piezoelectric device, a thin film transistor device, a thin film diode, a resistance device, a switching device, a micro-magnetic device, and a micro-optical device.
In an embodiment, the devices are produced on the first substrate.
In an embodiment, the step of holding the devices on the temporary holding member includes forming a wiring portion on each of the devices.
In an embodiment, the wiring portion includes an electrode pad.
In another embodiment of the present invention, an image display unit is provided. The image display unit includes an array of a number of light emitting devices each having an occupied area mounted on a wiring board for displaying an image in response to an image signal, wherein the occupied area ranges from about 25 xcexcm2 to about 10,000 xcexcm2.
In an embodiment, a ratio of the occupied area of each of the light emitting devices to an occupied area of a pixel on the image display unit ranges from about 10 to about 40,000.
In an embodiment, the ratio ranges from about 10 to about 10,000.
In an embodiment, the light emitting device is selected from the group consisting of a nitride semiconductor light emitting device, an arsenide semiconductor light emitting device, and a phosphide semiconductor light emitting device.
In an embodiment, a pixel includes a set of three of the light emitting devices, wherein each of the light emitting devices in the set has a different wavelength.
In an embodiment, a current retention circuit is electrically connected to each of the light emitting devices for retaining a current flowing in each of the light emitting devices.
In an embodiment, the current retention circuits are formed in a chip shape, and are mounted, together with each of the light emitting devices, on the wiring board.
In an embodiment, each of the current retention circuits formed in the chip shape has a size substantially equal to a size of each of the light emitting devices.
In a further embodiment of the present invention, a method of producing an image display unit having an array of a number of light emitting devices for displaying an image in response to an image signal is provided. The method includes the steps of preparing a wiring board on which wiring is provided in a matrix, preparing the light emitting devices separated into individual chips, wherein an occupied area of each of the light emitting devices ranges from about 25 xcexcm2 to about 10,000 xcexcm2, and mounting the light emitting devices on the wiring board so as to connect the light emitting devices to the wiring.
In an embodiment, the method further includes the steps of stacking a semiconductor layer on a device forming substrate, forming the light emitting devices on the semiconductor layer so as to be arrayed thereon, separating each of the light emitting devices into individual chips, and mounting the individual chips of each of the light emitting devices on the wiring board.
In an embodiment, the method further includes the steps of forming grooves reaching a front surface of the device forming substrate in a region adjacent to two of the light emitting devices so as to surround each of the light emitting devices, separating each of the light emitting devices surrounded by the grooves from the device forming substrate, and mounting each of the separated light emitting devices on the wiring board.
In an embodiment, the step of mounting each of the separated light emitting devices onto the wiring board includes positioning each of the separated light emitting devices such that at least one of a front surface and a back surface of each of the separated light emitting devices is attracted by an attracting mechanism.
In an embodiment, the step of separating each of the light emitting devices from the device forming substrate includes irradiating each of the light emitting devices with an energy beam from a back surface of the device forming substrate.
In an embodiment, the method further includes the steps of staging each of the light emitting devices on the device forming substrate between the device forming substrate and a temporary holding board before the step of separating each of the light emitting devices from the device forming substrate, and staging each of the light emitting devices on the temporary holding board after the step of separating each of the light emitting devices from the device forming substrate.
In an embodiment, an adhesive is substantially formed on the temporary holding board, and a front surface of each of the light emitting devices is temporarily affixed to the adhesive.
In an embodiment, the step of mounting each of the separated light emitting devices on the wiring board includes pressing an electrode portion of the light emitting device into contact with a conductive material on the wiring board.
In yet another embodiment of the present invention, a method of producing light emitting devices is provided. The method includes the steps of applying a semiconductor layer onto a substrate, forming an array of a number of light emitting devices on the semiconductor layer, separating the array of light emitting devices into individual light emitting devices, and separating each of the light emitting devices from the substrate.
In an embodiment, the step of separating each of the light emitting devices from the substrate includes irradiating each of the light emitting devices with an energy beam from a back surface of the substrate.
In an embodiment, the method further includes the steps of staging each of the light emitting devices on the substrate between the substrate and a temporary holding board before the step of separating each of the light emitting devices from the substrate, and staging each of the light emitting devices on the temporary holding board after the step of separating each of the light emitting devices from the substrate.
In an embodiment, an adhesive material is substantially formed on the temporary holding board, and a front surface of each of the light emitting devices is temporarily affixed to the adhesive.
In another embodiment of the present invention, a method of producing an image display unit on which devices are arrayed in a matrix is provided. The method includes the steps of transferring the devices arrayed on a first substrate each having a first pitch to a temporary holding member such that the devices are spaced from each other with a second pitch that is larger than the first pitch of the devices arrayed on the first substrate, staging the devices on the temporary holding member, transferring the devices to a second substrate such that the devices are spaced from each other with a third pitch that is larger than the second pitch, and forming wiring to be connected to each of the devices.
In an embodiment, the devices include at least one of a light emitting device and a liquid crystal control device.
In an embodiment, a pixel includes a set of at least a portion of the devices which each correspond to a different wavelength.
In an embodiment, the step of staging the devices on the temporary holding member includes forming an electrode pad on each of the devices, and the step of forming wiring to each of the devices includes forming wiring to the electrode pad.
In a further embodiment of the present invention, an image display unit is provided. The image display unit includes a wiring board and a number of light emitting devices mounted on the wiring board along a principal plane of the wiring board, wherein the light emitting devices include a crystal growth layer formed during crystal growth in an inverted direction that is perpendicular to the principal plane.
In an embodiment, a portion of the crystal growth layer is formed from a substrate via a window portion, and the light emitting device is separated from the substrate before being mounted on the wiring board.
In an embodiment, the image display unit further includes a first conductive layer, an active layer, and a second conductive layer formed on the crystal growth which has a crystal plane that is inclined with respect to a principal plane of the substrate, and a first electrode connectable to the first conductive layer and a second electrode connectable to the second conductive layer, wherein a height of the first electrode is substantially equal to a height of the second electrode.
In an embodiment, the image display unit further includes a first conductive layer, an active layer, and a second conductive layer formed on the crystal growth layer which has a crystal plane that is inclined with respect to a principal plane of the substrate, and a first electrode connectable to the first conductive layer and a second electrode connectable to the second conductive layer, wherein the crystal growth layer is positioned between the first electrode and the second electrode in a direction perpendicular with respect to a principal plane of the substrate.
In an embodiment, the crystal growth layer includes a nitride semiconductor composed of wurtzite formed by selective crystal growth.
In an embodiment, the crystal growth layer is formed by selective growth into at least one of a hexagonal pyramid shape and a hexagonal trapezoid shape.
In yet another embodiment of the present invention, a method of producing an image display unit is provided. The method includes the steps of producing a light emitting device on a substrate by selective growth of a crystal growth layer via an opening on the substrate, forming a first conductive layer, an active layer, and a second conductive layer on the crystal growth layer, forming a first electrode connected to the first conductive layer and a second electrode connected to the second conductive layer such that a height of the first electrode is substantially equal to a height of the second electrode, separating the crystal growth layer from the substrate, and mounting the crystal growth layer on a wiring board in a position that is inverted with respect to a position of the crystal growth layer on the substrate.
In an embodiment, a connecting portion is connected to at least one of the first and second electrodes such that the heights of the first and second electrodes are substantially equal.
In an embodiment, the step of mounting the crystal growth layer on the wiring board includes mounting the light emitting device on the wiring board such that at least one of a front surface and a back surface of the light emitting device is attracted by an attracting mechanism.
In an embodiment, the step of separating the crystal growth layer from the substrate includes irradiating the light emitting device with an energy beam from a back surface of the substrate.
In an embodiment, the irradiating with the energy beam is selectively performed.
In an embodiment, the method further includes the steps of holding the light emitting device between the substrate and a board for transfer before the step of separating the crystal growth layer from the substrate, and holding the light emitting device on the board for transfer after the step of separating the crystal growth layer from the substrate.
In another embodiment of the present invention a device mounting board is provided. The device mounting board includes a wiring board and a number of devices attached to the wiring board along a principal plane of the wiring board, wherein the devices include a crystal growth layer formed during crystal growth in an inverted direction with respect to crystal growth that is perpendicular to the principal plane.
In an embodiment, each of the crystal growth layers includes a portion extending along a substantially flat surface plane such that each portion has a substantially identical height relative to a surface of the wiring board upon which the devices are mounted.
Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the Figures.